An engobe is a slip with some glaze-forming material added, which puts it halfway between the two. That small change buys a wider application window than plain slip: an engobe can usually be applied to drier clay, and often to bisque, without the flaking that would ruin a plain coloured slip.
What the added flux does
Adding frit or feldspar to a slip lowers its shrinkage and makes it partially vitrify in the firing. Less shrinkage means less stress between the coating and the body underneath, so it holds on clay that has dried further than slip would tolerate. Partial vitrification means the surface is harder and less absorbent than a raw slip, and often needs no glaze at all for decorative work.
When to reach for one
When you want colour on a piece that has already dried past leather hard, when you want a coloured surface that can be left unglazed, or when a slip keeps flaking. Engobes also suit large flat surfaces where a plain slip is most likely to lift, such as tiles and slab work.
Applying and firing
Brushed, dipped, sprayed or trailed, in two or three coats for solid colour, the same as slip. The firing range is printed on the product and it matters more than for a slip, because an engobe with too much flux for the temperature becomes a glaze and runs. Test on a tile at the temperature you fire to before committing to work.
Buying against making
Commercial engobes are sold in ranges of colours, ready mixed or dry, and are the practical route for most people. Making your own means adding frit to a base slip in small increments and testing, which is worth doing if you are already mixing your own slips and have a specific problem to solve, such as a colour that keeps flaking off tiles.
Questions people ask about engobe
What is the difference between slip and engobe?
An engobe is a slip with glaze-forming material added, so it shrinks less, sticks to drier clay and partially vitrifies in the firing.
Can an engobe be used on bisque?
Often yes, which is one of the main reasons to choose one. Check the product's own guidance, because formulations vary.
Does an engobe need a glaze over it?
Not for decorative work, since it partially vitrifies. For functional ware a clear glaze still seals it properly.